Field of the Invention
The invention relates to a printed circuit board and more particularly to a printed circuit board design with good signal quality.
Description of the Related Art
Typically, a printed circuit board (PCB) is a layered insulating structure with internal and external traces/planes that allows electronic devices/components on the printed circuit board to be electrically connected to each other and to the outside environment. Printed circuit boards are the most commonly used packaging medium for electronic circuits. Due to the demand for high performance systems, packaging density and propagation speeds have increased, to force the technological development of the printed circuit boards to evolve from being single-sided to double-sided or multilayered.
Notwithstanding, along with rapid advances in printed circuit board technology, electromagnetic interference (EMI) problems have increased in severity. When electronic devices/components operate in high speed and are allocated in the PCB with a high device density, noise may be generated and affect the operation of other electronic devices. In a good printed circuit board design, signal delay, distortion and crosstalk noise are minimized. Crosstalk is a kind of noise induced primarily by the electromagnetic coupling between signal lines and degrades signal quality. In printed circuit boards, crosstalk occurs by the electrical coupling between nearby signal traces. As more and more functions are integrated on a chip, more circuit traces of the printed circuit board are demanded, and thus the coupling between nearby signal traces have become greater, introducing noise and false signals into systems.
Additionally, referring to FIGS. 8A and 8B, FIG. 8A shows a plan view of a conventional two-layered printed circuit board 300 which shows routing of power and signal planes. FIG. 8B shows a cross section along line 8B-8B′ of FIG. 8A. The conventional printed circuit board 300 has a top layer on a top surface 102 of a substrate 100, covered by a solder mask layer 126. The top layer comprises power traces 108a and 108b and a signal trace 112. The conventional printed circuit board 300 also has a bottom layer covered by a solder mask layer 130 comprising a ground plane 140 on a bottom surface 103 of the substrate 100. For descriptive convenience, the solder mask layer 126 is not shown in FIG. 8A. The power traces 108a and 108b are used for providing power potential, and the signal trace 112 is used for transmitting signal or data. As shown in FIG. 8A, the signal trace 112 is substantially placed along a second direction 304 which may form a power transmission barrier between the adjacent power traces 108a and 108b which is substantially placed along a first direction 302, which is not parallel to the second direction 304. As shown in FIG. 8B, in order to transmit power potential between the adjacent power traces 108a and 108b, a conductive trace 108c is formed on a bottom surface 103 of the substrate 100. The conductive trace 108c is respectively electrically connected to the power traces 108a and 108b by via plugs 134 through the substrate 100, and isolated from a split ground plane 140 by a gap 150. As shown in FIG. 8A, the signal trace 112 is across over the gap 150 surrounding the conductive trace 108c. When signals, especially high-speed signals, are transmitted along the signal trace 112, however, the current return path of the high-speed signals not only remains under the signal trace 112, but also transmit along the gap 150 directly under the signal trace 112. The long current return path may generate an undesired magnetic field vertical to the directions 302 and 304, and thus increases the coupling coefficient between adjacent signal traces and exacerbates electromagnetic interference (EMI) problems. A multi-layered PCB, which separates power, signal and ground planes in various layers, may be used to mitigate the aforementioned problems, but increasing layers of the PCB will also raise the manufacturing cost of the printed circuited board.
Accordingly, there is a need to develop printed circuit board designs which are capable of mitigating the aforementioned problems.